Tuning the Growth of Chiral Gold Nanoparticles Through Rational Design of a Chiral Molecular Inducer

The bottom-up production of chiral gold nanomaterials holds great potential for the advancement of biosensing and nano-optics, among other applications. Reproducible preparations of colloidal nanomaterials with chiral morphology have been reported, using cosurfactants or chiral inducers such as thiolated amino acids. However, the underlying growth mechanisms for these nanomaterials remain insufficiently understood. We introduce herein a purposely devised chiral inducer, a cysteine modified with a hydrophobic chain, as a versatile chiral inducer. The amphiphilic and chiral features of this molecule provide control over the chiral morphology and the chiroptical signature of the obtained nanoparticles by simply varying the concentration of chiral inducer. These results are supported by circular dichroism and electromagnetic modeling as well as electron tomography to analyze structural evolution at the facet scale. Our observations suggest complex roles for the factors involved in chiral synthesis: the chemical nature of the chiral inducers and the influence of cosurfactants.


Gold seeds for preparation of gold mini-rods
To a glass vial, 5 mL of stock solution #1 (50 mM CTAB, 13.5 mM n-decanol) was transferred before addition of 50 μL HAuCl 4 solution (aq., 50 mM) under constant magnetic stirring.After thorough sonication, 25 μL AA solution (aq., 100 mM) was added under constant magnetic stirring (200 rpm) until the solution became colorless.The stirring was then increased to 1500 rpm, and a 200 μL aliquot of NaBH 4 solution (aq., 20 mM) was quickly added by rapidly depressing the plunger of the micropipettor; the solution turned a deep brown color.Gold seeds were prepared in triplicate, and UV-Vis spectra were analyzed for reproducibility and target peaks (350 and 480 nm) indicate the formation of Au seeds of around 1.5 nm, as described in the literature. 2 Au seeds were rested at room temperature for at least one hour, but not more than four hours, prior to further use.

Gold mini-rod seeds for large gold nanorod (AuNR) preparations
In a glass vial, 80 mL of stock solution #1, 640 μL of AgNO 3 solution (aq., 10 mM), and 4.4 mL of HCl solution (aq., 1 M) were mixed before 800 μL HAuCl 4 solution (aq., 50 mM) was added under constant magnetic stirring.After thorough sonication, 1040 μL AA solution (aq., 100 mM) was added under constant magnetic stirring (200 rpm) until the solution became colorless.The stirring was then increased to 600 rpm, and 4.8 mL of Au seed dispersion was added; after an overnight incubation at 25 °C, the solution turned a brownish color.UV-Vis spectra were analyzed for a target LSPR peak of 734 nm, which corresponds to particle dimensions of ca.21 nm  7.5 nm.Initial centrifugation at 13000 rpm for 30 minutes was used to remove larger particles; the supernatant was then subjected to three additional rounds of centrifugation at 14500 rpm for 60 minutes.After each round, the supernatant was discarded and the particles resuspended in 10 mM aqueous CTAB solution.

Gold nanorods (AuNRs) for chiral preparations
In a glass vial, 80 mL of stock solution #2 (50 mM CTAB, 11 mM n-decanol), 9.6 mL HCl solution (aq., 1 M), and 1.2 mL AgNO 3 solution (aq., 50 mM) were mixed before 800 μL HAuCl 4 solution (aq., 50 mM) was added under constant magnetic stirring.After thorough sonication, 640 μL ascorbic acid solution (aq., 100 mM) was added under constant magnetic stirring (200 rpm) until the solution became colorless.The stirring was then increased to 600 rpm, and 7 μL of cleaned gold mini-rod seeds ([Au 0 ] = 29.8mM) was added; after an overnight incubation at 16 °C in a temperature-controlled water bath, the solution turned a reddish-brown color.Synthesized nanorods were purified by three rounds of centrifugation (7500 rpm, 30 min); after each round, the supernatant was discarded, and the particles redispersed in 1 mM CTAC.To assist removal of CTAB and Ag + ions, nanorod dispersions were heated at 60 °C for 30 min before each round of centrifugation.

Use of large gold nanorod seeds for chiral preparations
Chiral preparations were made with two different batches of Au NR seeds (Figure S1).

Chiral gold nanorods
Chiral Au NRs were prepared in 2 mL reaction volumes at 40 °C; CTAC and AA concentrations were kept constant at 44 mM and 700 mM, respectively, and [Au 3+ ]:[Au 0 ] was kept constant at 8.9 (185 μM and 20.8 μM, respectively).For all chiral preparations, and after a 30 min incubation, chiral nanorods were spun down (3250 rpm, 10 min) and resuspended in Milli-Q water.This process was repeated twice to remove excess AA and CTAC from solution.
Considering the average dimensions of the Au NR seeds (142 nm x 31.6 nm) and assuming a cylindrical shape, the surface area per NR would be SA = 15665 nm 2 and the volume per NR V = 111366 nm 3 .Considering the initial Au NR seed concentration of 2.08  10 -5 mol/L, this would translate into 1.90476  10 12 Au NR/L, with a total SA T = 6.0  1013 nm 2, in 2 mL of dispersion.
For each LipoCYS molar concentration, the number of molecules can be readily calculated as: LipoCYS molecules = [LipoCYS] (mol/L) vol.dispersion (L) x N A (molecules / mol), where N A is the Avogadro number.Again, for 2 mL: 20 μM  2. 4

Electron tomography
High resolution scanning transmission electron microscopy (STEM) and electron tomography experiments were performed using a "cubed" Thermo Fisher Scientific Themis Z instrument operated at 300 kV in HAADF-STEM mode.High angle annular dark field STEM (HAADF-STEM) tomography series were acquired over an angular range of ±75°, with a tilt increment of 3°, and were used as an input for SIRT reconstruction using the Astra Toolbox 1.8 for MATLAB R2019A. 3Electron diffraction tomography (ED) series was acquired over an angular range of ±75°, with a continuous tilting (~0.6°/s) and were reconstructed using the PETS2.0software.Visualization of the 3D reconstructions was performed using the Amira 5.4.0 software.compared with other approaches.SIE-MoM is robust against instabilities produced by rapid spatial variations of the permittivity, as is usually the case in plasmonic structures.
Gold was described through its frequency-dependent complex permittivity, taken from optical measurements. 8,9noparticle Models: The nanorod models were designed to resemble the experimental 3D tomographic reconstructions of NPs obtained using 20, 45 and 90 μM LipoCYS.The models consist of a twisted nanorod core with square section (70 nm  70 nm) and 170 nm in length, on which 16 or 25 helical grooves are excavated all around (for the second and third models, respectively), giving rise to 6 or 16 helical wrinkles that unfold along the lateral surfaces.The helices have two leveled and two inclined steps per pitch, with tilt angles of 60 º and 30 º in the inclined steps, respectively.The corresponding wrinkle widths are 8 nm and 4 nm, and the separation distances between wrinkles are 12 nm and 7 nm, respectively.Groove depths are 6 nm and 13 nm, respectively.The modeling and meshing of the models were performed with Blender 3.3, Solidworks and Hypermesh respectively.Having a high-quality, clean mesh, with a consistent aspect ratio across all elements is crucial for the results of the simulation.Mesh quality is key in obtaining accurate results and for the problem to be solved efficiently in terms of time and resources, and with fidelity to the underlying physics.Illumination was based on left-(right-) circularly polarized plane waves impinging from multiple directions.Overall responses were calculated by averaging over light incidence angles.

Figure S2 .
Figure S2.Isosurface visualizations of the 3D reconstructions (i, ii) for Au NRs obtained using different concentrations of (S)-LipoCYS (top) and (R)-LipoCYS (bottom) (A,D: 20 μM; B,E: 45 μM; C,F: 90 μM).Presented images are made along different viewing angles (oriented 45° relatively to each other) for each particle.Plots of the corresponding helicity function (iii; red: right-handed; blue: left-handed) are provided for each 3D reconstruction.Technical details on how to compute these helicity functions are provided in the "Electron Microscopy" section (page S14).The Au NRs obtained by (S)-LipoCYS are found to yield a left-handed helicity (blue), whereas those obtained by (R)-LipoCYS are right-handed.All scale bars are 25 nm.

Figure S3 .
Figure S3.Isosurface visualizations of the 3D reconstructions (i,ii) for Au NRs obtained using 200 μM of (S)-LipoCYS (A) and (R)-LipoCYS (B).Presented images are made along different viewing angles (oriented 45° relatively to each other) for each particle.(iii) Plots of the corresponding helicity function are provided for each 3D reconstruction.Technical details on how to compute these helicity functions are provided in the "Electron Microscopy" section.A less defined helicity plot was obtained, which is reflected in the equal appearance of red and blue features in both panels iii, indicating undefined chirality.

Figure S4 .
Figure S4.Electromagnetic simulations for 3D models (SolidWorks/HyperMesh) of chiral nanorods synthesized with 20 (top), 45 (middle) and 90 μM (bottom) (R)-LipoCYS.For each model: simulated g-factor spectra and cross-section spectra for a randomly oriented distribution of nanorods, under illumination with left-and right-handed circularly polarized light (cross-section spectra are shown as an average for left-and rightcircular polarizations).

Figure S5 .
Figure S5.Electromagnetic simulations for a 3D model of Au NRs with disordered surface features, resulting in strongly decreased g-factor.Spectra and cross-section spectra are shown for randomly oriented distributions of nanorods, under illumination with left-and right-handed circularly polarized light (cross-section spectra are shown as an average for left-and right-circular polarizations).

Figure S6 .
Figure S6.Morphological characterization of Au NPs obtained by increasing the concentration of (S)-LipoCYS (A: 20 μM, B: 45 μM, C: 75 μM, D: 90 μM) during chiral overgrowth.The morphological characterization for each sample includes: (i) HAADF-STEM image of several representative nanoparticles; (ii, iii) Visualizations of the 3D reconstructions presented along different viewing angles (oriented 45° relative to each other); (iv, v) selected orthoslices extracted from the 3D reconstructions, perpendicular to the longitudinal and transverse axes, at the center of the NRs.White dashed lines represent the relative positions of slices shown in (iv) and (v).All scale bars are 25 nm.

Figure S7 .
Figure S7.Selected orthoslices extracted from the 3D reconstructions, perpendicular to the longitudinal and transverse axes, at the center of chiral Au NRs prepared with different concentrations of of (R)-LipoCYS (a,d: 45 μM, b,e: 75 μM, c,f: 90 μM).All values are given in nm.

Figure S8 .
Figure S8.(A,D) HAADF-STEM images and (B,C,E) high resolution HAADF-STEM images of a chiral AuNR prepared with (R)-LipoCYS (similar to that in Figure 3A), taken along [110] and [100] zone axis.The insets in (B), (C) and (E) show fast Fourier transform (FFT) patterns along [110] and [100] directions for the fcc lattice of Au, which indicate that the tips are enclosed by {110} facets.

Figure S9 .
Figure S9.(A) Idealized surface morphology of a NP obtained using 20 µM (R)-LipoCYS, indicating that concave lateral facets of a NP consist of facets of the {520} family.(B) Electron tomography reconstruction of a NP obtained using 20 µM LipoCYS, tilted such that the viewing direction is parallel to the [011] direction, indicating twisted features forming around <011> and <111> corners (dashed black lines), similar to the particles reported by Lee et al. 10 Scale bar: 25 nm.